To properly drive a heat engine by a lean fuel without making the device large.

This device comprises a lean fuel supply means 3 for supplying lean fuel such as reducing gas or methane gas to a heat engine 2; an auxiliary fuel supply means 4 for supplying auxiliary fuel such as propane gas or light oil to the heat engine 2; an actuation condition detecting means (heating value detecting means) 6 for detecting the actuating condition of the heat engine 2; and a proportion control means 24 for controlling the proportion of the lean fuel supplied from the lean fuel supply means 3 to the heat engine 2 and the auxiliary fuel supplied from the auxiliary fuel supply means 4 to the heat engine 2 according to detection signals of the actuating condition detecting means 6.